The present disclosure is directed to a particular kind of geophone spread and more particularly a set of geophones adapted to be used in a downhole enironment. Ordinarily, seismic data is collected by placing a shock or impulse source on the surface, creating a shock wave which is propagated through the earth, reflected from underground horizons, and detecting small signals for recordation from a geophone spread. The typical geophone spread includes any number of geophones which are placed on the ground. The several geophones are connected by long cable to truck located equipment which includes individual geophone amplifiers, data or signal conditioning circuits, and a recorder with a timing reference placed on the recorded data.
In the past, geophones have also been placed in a well borehole near a spread of geophones. Sometimes, the geophones may be placed in the well borehole simultaneously with recording data from surface located geophones. Sometimes, the data can be collected on two different shots so that the data observed downhole is obtained prior to or after the collection of data from the geophone spread.
It is necessary to provide a reference which is affixed to the geophones so that the orientation in space and location along the borehole can be determined. If this orientation cannot be determined, then the data obtained from the geophones may be difficult to fix in space relative to the adjacent formations.
The present apparatus and methods set forth a means and procedure for determining location of the geophones in any vertical or deviated well. The present invention apparatus contemplates the attachment of a reference determining system to the geophone apparatus. The geophones are thus placed in a hermetically sealed housing. This protects the geophones from invasion of drilling fluid. This housing is attached to another housing which defines a north seeking reference. The north seeking reference is constructed with and made of a north seeking gyroscope and cooperative accelerometer system. Thus, the equipment may "tumble" as it moves to any relative orientation in the well. Even though it may take any angular position in space, it is not blind to its orientation. That is, it is able to locate its orientation relative to a particular reference system (north of the earth in this instance) and is therefore able to provide a azimuthal reference relative to the set of geophones.
Geophones however are extremely sensitive devices. That is, they are built to receive extremely small vibrations traveling long distances through the earth and provide electrical signals indicative of such vibrations. The present apparatus thus incorporates a gyroscope system which is able to be switched off completely. When it is off, data can then be recorded. The gyroscope is then switched on, brought to operating speed, locates north as a reference, and provides an indication of the orientation in space of the gyroscope relative to the reference. In turn, this reference information defines the orientation of the geophones in the equipment. When the geophones are oriented, the data obtained by the geophones can then be properly related to the seismic source and formations of interest. This enables precise determination of the three dimensional position and orientation of the geophones in the well and proper correlation between the data obtained from geophones in the borehole.
The present apparatus is preferably constructed as a single unit adapted to be lowered on a cable having conductors therein. The equipment includes a telemetry section which properly conditions the signals for transmittal to the surface along the cable. An XYZ geophone system is also included. The apparatus includes a stable platform having the form of a gyroscope. This particular gyroscope is the type which can be switched off and on, yet when switched on, can locate a particular reference, preferably north, thereby providing a reference whereby the instrument can be located and orientation determined in the borehole. The logging cable that connects with this apparatus extends to the surface and the depth of the cable in the well is measured also. The various measurements enable precise location and orientation of the geophones so that the relative location of the instrument in the borehole can be properly correlated with other known measurements.